CU alumni compare experimental climate change to human trampling on biocrusts, with disturbing results.
The potential effects of climate change are just as bad as human trampling for biological soil crust communities, two University of Colorado Boulder alumni have found.
The findings were published this fall in the prestigiousÌęProceedings of the National Academy of SciencesÌęby CU-Boulder alumni and U.S. Geological Survey researchers Scott Ferrenberg (EEB â14) and Sasha Reed (EEB â08).
One of the longest-running studies of its kind, Ferrenberg and Reedâs 15-year study in Moab, Utah, compared the effects of foot traffic to the expected impacts of climate change on biological soil crust communities on the semiarid Colorado Plateau.
Scott Ferrenberg, who earned his Ph.D. in biology from CU-Boulder, is shown near Moab, Utah, where he and CU alumna Sasha Reed conducted a groundbreaking study on biocrust. Photo courtesy of Scott Ferrenberg.
Soil crusts are made of cyanobacteria, more commonly known as algae. Mosses and lichens are part of the biological soil crust community as well. These crusts glue the soil together through complex sugars.
Vital for soil fertility and stability, biocrusts are threatened by physical impact from foot and vehicle traffic. Physical destruction of biocrusts kills the lichens and mosses, which then harms soil fertility, and increases dust storms and health hazards. These effects have been well documented by ecologists around the world.
Ferrenberg and Reed compared the effects of physical pressure to the potential effects of climate change by exerting physical force on a control plot and subjecting other plots to experimental climate change. They continually warmed five plots of land a few degrees above the surrounding area, increased the frequency of watering on another five plots, and then both artificially warmed and watered a third set of plots.
By simulating the projections for climate change in the American Southwest, Ferrenberg and Reed hoped to learn what is in store for the semiarid landscapes of Colorado and Utah, and what the global implications of climate change might be.
As expected, the lichens and mosses that endured human trampling in the control plots were immediately damaged, but âwhat was sad and shocking to us was that giving water and warming had the same effect as physical pressure,â says Ferrenberg.
Effects of increased precipitation
In accordance with water predictions for the future of the Southwest, Ferrenberg and Reed increased the frequency of small rainfall events for one set of plots, meaning they frequently gave them small amounts of water, âjust enough to get the surface noticeably wet,â Ferrenberg says.
They hypothesized that the extra water would either benefit or not change the biocrusts much.
âYouâd think giving water to stuff in the desert would be good, but it wasnât,â Ferrenberg says.
In fact, it ultimately destroyed the mosses and even the lichens in the first year of the experiment.
Site of the biocrust experiment.
âThese organisms have adapted to the pulses of moistureâŠBecause itâs [normally] such a rare event to get moisture, the crusts will become active with any amount of water,â Ferrenberg says. âThese plants have no vascular system, so as soon as [the mosses] get wet, they start photosynthesizing.â
Unlike regular plants, biocrust plants can photosynthesize only when wet, and they have evolved to become extremely active to take advantage of the limited rainfall in the desert.
âBiocrust organisms have been so successful because they become active really quickly,â Reed says.
With frequent, small rainfall events, though, this trait becomes negative.
When there isnât enough water to complete photosynthesis, mosses put out too much energy without getting any back. Forced to endure too many small rainfall events, these crusts ultimately starve themselves.
There are studies from a number of other countries, as well as the work here in the United States, to suggest future problems for the survival of these crusts in the face of climate change."
âPretend youâre a moss and you get wet and immediately gear up to photosynthesize,â Reed says. âIt costs you energy; youâre eating through carbon.â
The physiological mechanism that helps them survive in the desert drove them into carbon starvationâin which water-starved plants stop photosynthesizingâwithin one year of the small rainfall event treatment.
âThey are able to cope with the demands of photosynthesis once or twice, but not over and over,â Ferrenberg says.
If the climate projections for precipitation in the Southwest are correct, mossesâ and lichensâ ability to survive will actually kill them, leaving a less-complex biocrust to stabilize and fertilize soil.
Effects of increased temperature
Using infrared heating lamps suspended over the surface, Ferrenberg and Reed warmed another set of plots a few degrees higher than the surrounding land. This mimicked the increased temperatures projected 100 years from now.
âIncreased temperatures caused a slow, but inevitable decline after about six or seven years,â Ferrenberg says.
Reed says they are not yet sure of the physiological mechanism that caused the mosses to respond so badly to the increase in heat, but she says it likely has to do with increased stress.
The fourth set of plots received watering and warming treatments together.
Reed says this was bad for mosses, but was âby far the worst for the lichens.â
Impacts of degraded soil crusts
While physical destruction has an immediate, devastating impact on biocrusts, it is preventable, in that it is possible to keep people off biocrusts.
Stopping climate change is another matter.
âWe canât stop warming at this time, so weâre going to see a degradation of crust,â Ferrenberg says.
Healthy biocrust community.
The effects of climate change, then, will create less stable soil. This increasingly eroded soil ends up in river ways and disturbs dust that is then exported internationally.
This is problematic for the western United States, which has many deserts and biocrusts, but the problem doesnât end there. Biocrust is a main component of dryland ecosystems, which make up 40 percent of the earthâs land surface.
Ferrenberg and Reed represent just one group trying to determine how universal these effects will be.
âThere are studies from a number of other countries, as well as the work here in the United States, to suggest future problems for the survival of these crusts in the face of climate change,â Reed says.
One big question theyâre trying to address is whether biocrusts in hot deserts, like the Chihuahuan desert in southern New Mexico, are different from those in âcold desertsâ like the Colorado Plateau in Moab.
âFor example, one hypothesis could be that hot desert crusts are less vulnerable to the type of climate change we expect, and thusâŠcold deserts around the worldâŠcould face a bigger problem,â Reed says.
Lara Herrington Watson is a CU alumna (â07) and freelance writer who splits her time between Denver and Phoenix.
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